Filters used for ultrafiltration and reverse osmosis include spiral types wherein a flat separation membrane is coiled in spiral fashion, types wherein a plurality of hollow fiber separation membranes are arranged in parallel and tubular types wherein a flat separation membrane is worked into a cylindrical shape, and all such types can be held in a cartridge with a fixed volume for use.
Among such separation membranes, the flat separation membranes are produced as sheets by coating a resin with a separating function onto a support substrate such as a nonwoven fabric. The nonwoven fabric used as the support substrate functions as a coating base for production of a uniform film when the separation membrane is produced, while also performing the basic function of strength maintenance to prevent rupture of the separation membrane due to the pressure of the filtration medium during use. Therefore, a staple fiber wet laid nonwoven fabric is used which can provide a high degree of uniformity.
In recent years, as such separation membranes have become more widely used, an improvement in treatment efficiency, per cartridge, has also become an important issue. Thinner separation membranes including support substrates have therefore been desired, in order to maximize the number of separation membranes that can be placed in each cartridge and lower the pressure loss of the separation membranes for an increased flow volume.
In order to achieve thin, uniform coating films, the support substrate itself must be reduced in thickness while maintaining surface smoothness and strength. However, when the amount of fiber is reduced to form a thinner support substrate, the problem of “strike-through” can occur whereby the resin seeps out from the back of the support substrate during coating. The resin escaping from the back side of the support substrate contaminates the film-forming apparatus and can cause defects in a continuously formed separation membrane.
Also, although reducing the amount of fiber to form a thinner support substrate increases the liquid permeability, it also results in noticeable thickness spots in the support substrate and more sections susceptible to “strike-through”. Another serious problem is reduced strength and, particularly in the case of a staple fiber wet laid nonwoven fabric, reducing the amount of fiber results in a drastic reduction in strength. Methods exist for increasing the apparent density to achieve thinner size but, as the fiber diameter (D) and fiber length (L) must have a ratio (L/D) within a specified range in order to achieve a uniform dispersion to avoid tangling of the fibers in the wet laid stock solution, the fiber length must be shortened, thereby drastically reducing the strength of the nonwoven fabric.
In Japanese Unexamined Patent Publication No. 2002-095937 and U.S. Pat. No. 6,156,680 there are proposed methods of using low-crystalline polyethylene terephthalate staple fibers and methods of combining low-melting-point fibers for the purpose of increasing a heat bonding strength. However, for the reasons explained above, the strength is significantly low when the fiber diameter is 4.5 μm or smaller and, therefore, it has not been possible to sufficiently achieve a thinner size while preventing strike-through.
US2005-6301 describes a method of mixing staple fibers with different fiber lengths, but it is not possible to obtain a thin support substrate having sufficiently high strength while preventing strike-through.
Japanese Unexamined Patent Publication SHO No. 60-238103 proposes using a nonwoven fabric with a loose-dense bilayer structure prepared by a wet laid method, in order to better prevent strike-through of the coating resin. The loose-dense bilayer structure comprises a loose layer with a fiber diameter of 17-54 μm and a fiber length of 3-50 mm on the resin coating side and a dense layer with a fiber diameter of 2.7-17 μm and a fiber length of 3-50 mm on the back side.
However, a dense layer composed of thin fibers has a high L/D ratio and therefore the fibers tend to tangle together during wet laid, and the obtained nonwoven fabric is susceptible to defects such as projections. When the fiber lengths are shortened to reduce tangling between the fibers, the strength tends to be reduced.
The aforementioned Japanese Unexamined Patent Application No. 60-238103 therefore proposes a loose-dense-loose trilayer structure wherein a thick fiber layer is further situated on the back of the dense layer in order to guarantee strength. However, the obtained support substrate cannot be satisfactory because of increased thickness.
Japanese Unexamined Patent Publication SHO No. 61-222506 describes lamination of a nonwoven fabric formed by a staple fiber dry method and a melt blown nonwoven fabric, followed by heat bonding to form a loose-dense structure. Even by this method, however, the problem of unevenness is not eliminated in the staple fiber dry method. In addition, as a melt blown nonwoven fabric or ultrafine fiber wet laid nonwoven fabric has very low tensile strength and surface abrasive strength, for example, a fiber volume of 70 g/m2 or greater must be used in the case of ultrafine fibers, while a fiber volume of 100 g/m2 is necessary with a dry method nonwoven fabric. Consequently, the thickness of the support substrate is increased and a thin support substrate cannot be satisfactorily achieved.
Japanese Unexamined Patent Publication No. 2003-245530 proposes a separation membrane with improved strike-through prevention by providing gaps as the loose structure in the non-coating side, and using a thin nonwoven fabric with a thickness of no greater than 80 μm as the support substrate. The support substrate requires a large amount of resin for filling of the gaps in the loose layer when the coating resin penetrates from the dense layer to the loose layer, and the consequent effect of reduced penetration rate in the direction of thickness is utilized.
The aforementioned Japanese Unexamined Patent Publication No. 2003-245530 mentions staple fiber wet laid nonwoven fabrics with two different structures as examples of loose-dense structures. One of these is a nonwoven fabric having a structure produced using a calender with a temperature differential, for formation of a highly dense coating side with high adhesive strength by high-temperature bonding, and a low-density non-coating side having a uniform structure in the plane direction by low-temperature bonding, while the other is a nonwoven fabric having a structure which is irregular in the plane direction or periodically irregular, by forming hills and valleys on the non-coating side by emboss bonding.
However, problems with nonwoven fabrics having the former structure include weak bonding conditions in 50 wt % or more of the fibers, and wrinkles during the separation membrane production process due to a lack of strength or low rigidity. Because of weak bonding on the coating side, fluffing tends to occur upon contact with the guide roll in the resin coating step, leading to poor stability during resin coating.
A problem with nonwoven fabrics having the latter structure is that the valleys having high fiber density are resistant to penetration and the coating resin preferentially penetrates into the hills, such that the coating resin reaches the tops of the hills before penetrating to the valleys, therefore resulting in a non-uniform coating.
In order to avoid these problems, some supports are produced by lamination with perforated nonwoven fabrics or nonwoven fabrics with hill-valley forms produced by a separate step such as corrugation, but in such cases the hills and valleys cause thickness spots in the resin coating layer, resulting in reduced stability of the membrane performance. The thickness spots in the resin coating layer are also produced by emboss bonding, and are more notably produced with thinner nonwoven fabrics.
WO2004-94136 filed by the present inventors describes a support substrate composed of three layers: a thermoplastic filament nonwoven fabric, a melt blown nonwoven fabric and a thermoplastic filament nonwoven fabric, but the method of using the support substrate is not described in detail.